Turning machines, such as lathes, are widely used in industry to produce manufactured parts from bar stock of various shapes and materials. In general, a turning machine has a revolving spindle supported on a headstock of the machine. The spindle has a hollow center through which raw material in the form of bar stock is fed. The bar stock is held at the machining end of the spindle by a work holder fixedly attached to that end of the spindle, and the remainder of the bar stock extends through the hollow center of the spindle.
When the machine is used to perform operations on bar stock having a diameter that is only slightly smaller than the inner diameter of the spindle's hollow center, the bar stock is substantially supported by the spindle. However, if the diameter of the bar stock is too much smaller than the inner diameter of the spindle, the bar stock will be substantially unsupported by the spindle and may be subjected to bending and inertial forces which cause a whipping action of the unsupported length of the bar stock. This whipping action creates undesirable vibration at the machining end of the bar stock, making it difficult to machine properly by holding the required tolerances. Furthermore, the bar stock may contact the inside of the spindle, substantially increasing shop noise and potentially damaging the machine and the surface of the bar stock.
To address the problems associated with supporting and machining bar stock in turning machines, spindle liners (also referred to as filler tubes or reduction tubes) have been used to provide support of smaller size bar stock in machine spindles. Spindle liners are generally tubular in construction, with an outer diameter sized for a close slip fit into the spindle and an inner diameter sized for a close, clearance fit of the bar stock. Spindle liners are typically made from steel and are provided in various sizes to accommodate corresponding sizes of raw material.
Spindle liners can also be formed from polymeric materials, such as urethane. Polymeric spindle liners are convenient in that special shapes may be readily molded into the inner diameter to correspond to various shapes of bar stock, such as square or hex shapes. But these polymeric spindle liners have several drawbacks. For example, polymeric materials can “cold flow” under bolt clamping loads and thereby prevent a secure connection to a spindle flange. Because polymeric materials are not as strong as metal materials, the walls of a polymeric spindle liner must be made thicker than a corresponding metal spindle liner, especially when the length of the spindle liner extends beyond the end of the spindle. Having thicker walls diminishes the maximum capacity of bar stock which may be received by a polymeric spindle liner. Furthermore, machining the outer diameter of a spindle liner is generally more difficult when the material is polymeric. This can be problematic when a spindle liner must be re-worked to obtain a tight fit to a particular spindle's inner diameter, which can vary by as much as ±0.01 inch due to standard manufacturing tolerances.
A need exists for a spindle liner which has advantages of both metal and polymeric spindle liners and which solves various problems in the art such as those mentioned above.